Light irradiation therapy is used for the treatment of various blood diseases to, e.g., eliminate immunogenicity in cells, inactivate or kill selected cells, inactivate viruses or bacteria, or activate desirable immune responses. For example, it is known to use the photoactivatable compound 8-methoxypsoralen (8-MOP) to treat blood cells, such as lymphocytes, in an extracorporeal photopheresis (ECP) procedure. In one example of such a procedure, blood is withdrawn from the patient and the white blood cells, which are mononuclear cells, are separated (typically by centrifugation) from the remainder of the whole blood components. The separated white blood cells are combined with a selected dose of 8-MOP and subjected to light (typically UV-A) to activate the 8-MOP molecules. The light crosslinks 8-MOP to DNA strands inside the cell and on the cell wall of the exposed leukocytes, eventually causing cell apoptosis. The fluid with the altered white blood cells is reinfused back into the patient to induce an immune system response. Examples of a photopheresis method and system of the type described above are set forth in U.S. Patent Application Publication Nos. US2013/0197419 and US2014/0370491, the contents of both incorporated herein by reference in their entireties.
In order to administer an effective amount of activating light (e.g., UV-A) to the mononuclear cells, the mononuclear cell product must have a hematocrit and thickness that allows for the administered light to be effective in the treatment of the mononuclear cells. For example, if the hematocrit is too high—indicating too many red cells—the residual red blood cells may interfere with the light treatment, i.e., activation of the photoactivation agent, resulting in a less than fully treated mononuclear cell product. A conditioning solution, such as saline, may be used to increase the volume of and dilute the fluid in which the collected mononuclear cells reside to ensure that the correct thickness and hematocrit is reached in the container of cells to be treated. The photoactivation agent is likewise added to the mononuclear cells (e.g., in an irradiation container) and this mixture is irradiated in an illumination device with light. The treated cells are then returned to the patient.
The reinfusion of the treated cells may, however, also cause some undesired side effects in the patient. For example, the presence of unbound photoactivation agent in the patient's circulatory system may create certain discomforting conditions for the patient, such as light sensitivity and an increased susceptibility to sun burn. In addition to the potential side effects of unbound photoactivation agent, the amount of the conditioning solution used may be excessive and result in hypervolemia, which can be problematic for patients with cardiovascular problems. In mononuclear cell procedures in general, the amount of anticoagulant and saline returned to the patient is often much greater than the collected MNC volume, thereby causing a fluid imbalance in the patient. Therefore, it would be desirable to remove the excess conditioning solution and photoactivation agent from the treated cells, divert them and prevent them from being returned to the patient during an extracorporeal photopheresis procedure. The methods and systems described herein address this need.